This application claims the priority benefit of Taiwan application serial no. 88109893, filed Jun. 14, 1999.
1. Field of Invention
The present invention relates to an optical fiber grating package. More particularly, the present invention relates to a structure and manufacturing method for a passive temperature compensated fiber grating package.
2. Description of Related Art
Fiber grating can be used as a sensor in a wavelength measuring system. It also applies to wavelength division multiplexing (WDM), dispersion compensation, laser stabilization, gain flattening of optical amplifiers and so on, in a communication system. However, this is a problem in fiber grating applications. The Bragg wavelength of the fiber grating changes with the environmental temperature or external stress. In order to prevent the above conditions, several methods have been developed. For example, in an optical active system, the environmental temperature is dynamically maintained in a stable condition by a temperature control system, so as to maintain a stable central wavelength of the fiber grating. This active method has a drawback in that it consumes power when controlling the temperature. Another choice is to employ a passive system, which structure is less complex than the active system. The optical passive system can be made insensitive to temperature through a thermal compensation mechanism.
In the region where the fiber grating is located, the refraction index and the pitch of the fiber grating are inevitably affected by the temperature. The refraction index of the grating is usually very sensitive to temperature and thus hard to control. The control method in the passive system typically makes use of the expansion property of the material, which expansion property varies with the temperature. The central wavelength of the fiber grating therefore can be controlled by temperature. A ceramic material with a negative coefficient of thermal expansion (CTE) proposed by U.S. Pat. No. 5,694,503 is currently in use for compensation of the temperature effect in the passive system. This method has the advantages of small device dimensions and a simple structure, but it has at least one drawback in that the ceramic material is easily broken. Moreover, the negative CTE of ceramic material must be accurately controlled during fabrication.
In U.S. Pat. No. 5,042,898, two kinds of material with different but positive CTEs are used. The two materials are affixed together to form two tubes. Two ends of the fiber are respectively affixed to the two tubes of the materials. As temperature increases, the fiber length is loosened by the material with the larger CTE through release of the fiber so as to achieve thermal compensation. The central wavelength of the fiber grating therefore can be kept the same. The drawback of this method is that very high precision is necessary when affixing the fiber onto the two kinds of material. If such precision is not met, the compensation of expansion will fail.
Another method, using the plates made of two kinds of materials, is also proposed by patent WO98/27446. The fiber grating is fixed on the plate having a smaller CTE. As temperature increases, the plate becomes concave due to the difference of CTE between the two kinds of material, so that the fiber grating can be compensated. This method also has its drawback.
In the patent WO98/27446, quartz is used as the low CTE material, but it is difficult to bend quartz bends. As a result, temperature compensation is difficult to perform. Moreover, if the two plates are not properly fixed together, the thermal stress cannot be properly released. The reliability of the fiber grating package is reduced. In addition, a thin ion plate is used as the higher CTE plate . The temperature compensation of the thin ion plate is adjusted by its width. The device dimension cannot be effectively reduced.
In U.S. Pat. No. 5,841,920, a similar principle is applied in which two material elements with different CTEs and different geometric structures are used. By making use of the different CTEs and geometric structures, the material distorts to compensate for the fiber grating. This method still has a drawback in that an external impulse may shift the fiber grating, indirectly influencing the temperature compensation. Moreover, the method cannot allow adjustment of the fiber grating once the fiber grating is adhered, affixed or packaged on the element. This causes a low yield when a filter device is assembled.
As WDM transmission technology continues to develop, a precise and stable wavelength for transmission is essential. It becomes an important issue to have a precise and stable wavelength. The typical temperature coefficient of the fiber grating is about 0.01 nm/xc2x0 C., which is insufficient for the requirement in the WDM transmission. Moreover, as the channel number of the WDM increases, the channel spacing is reduced from 1.6 nm down to 0.4 nm. In the current WDM transmission technology, the wavelength precision is very important. The central wavelength of the fiber grating demands an error within +/xe2x88x920.025 nm. It is very important to precisely control the central wavelength of the fiber grating during packaging the fiber grating.
It is at least an objective of the invention to provide an improved structure for an optical fiber grating package, such that the packaged fiber grating and signals reflected or transmitted from the fiber grating are insensitive to the environmental temperature. The central wavelength of the fiber grating is therefore not affected by the environmental temperature. After packaging, the central wavelength of the fiber grating meets the specifications of the International Telecommunication Union (ITU). In addition, a method for fabricating the structure of the optical grating package is also provided so as to allow the structure to easily meet the specifications of the ITU. Adjustment of the central wavelength can be performed when the structure is on-line, resulting in a high yield of product.
The invention provides a structure for an optical fiber grating package. The structure includes a fiber grating that is mounted on a multi-layer metal plate. The fiber grating is formed on a fiber at the desired portion. The ends of the fiber grating are secured to the two ends of the multi-layer metal plate. The multi-layer metal plate includes, for example, a two-layer metal plate and a thinner metal plate on the two-layer metal plate. The two layers of the bimetal plate have different coefficients of thermal expansion (CTE). The thinner metal plate usually is, for example, about 10 times thinner than each of the two layers. The thinner metal plate is adhered to, for example, the one of the two layers with the greater CTE by a contact length, which is adjustable so as to compensate the CTE of bimetal plate. The structure further includes an adjusting plate located on the multi-layer metal plate at the side where the fiber grating is mounted so that the adjusting plate, serving as a pad, can lift the fiber of the fiber grating. The adjusting plate is also located in a position between the grating portion of the fiber grating and one fixed end. As a result, a grating pitch of the fiber grating can be further precisely adjusted by shifting the relative location of the adjusting plate to cause a necessary tension of the fiber. The multi-layer metal plate with the fiber grating is held by a substrate, such as metal substrate. The metal substrate is located in a tube casing, for example, for protection. Tube caps cover both ends of the tube so that the metal substrate is protected by the tube casing. Each cap includes, for example, an aperture to allow the fiber to pass through.
The invention also provides a method for fabricating the optical fiber grating package. The method includes providing a bimetal plate, which is planar at a packaging temperature of, for example, about 100xc2x0 C. This temperature is relatively higher than the normal environmental temperature. A thin metal plate is adhered to the bimetal plate at one side so that a multi-layer metal plate is formed and has a substantially flat structure at the packaging temperature. A desired contact length between the thin metal plate and the bimetal plate is set, according to design requirements for compensating the CTE of the bimetal plate. This structure is now called a multi-layer metal plate. A fiber grating portion of a fiber, whose central wavelength is adjusted to a desired value, is secured to the two ends of the multi-layer metal plate. An adjusting plate is inserted between the fiber and the multi-layer metal plate and is located between the fiber grating portion and one of the two ends. The adjusting plate is used to further adjust the central wavelength of the fiber grating. The multi-layer metal plate with the fiber grating is mounted on a substrate. A portion of the fiber other than the fiber grating portion is also attached to the two ends of the substrate with a reserved additional length, so that the fiber between the multi-layer metal plate and the ends of the substrate is not pulled due to a mechanical effect such as a distortion of the multi-layer metal plate as the environmental temperature changes. The substrate is further inserted in a tube casing, which is then covered with two end caps. Each end cap has an aperture through which the fiber passes.
The invention uses a multi-layer metal plate to hold a fiber grating so as to reduce the thermal effect on the fiber grating. An adjusting plate is also used to further adjust the central wavelength of the fiber grating. The central wavelength therefore remains stable as environmental temperature varies.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.